For example, as described in Patent Document 1 below, conventionally known seam welding is carried out by holding the surfaces of superimposed steel sheets between a pair of roller electrodes, applying pressure thereto, and rolling the two roller electrodes along the surfaces of the steel sheets while supplying current thereto, thus melting the steel sheets held between the two roller electrodes by virtue of Joule heating to carry out continuous welding.
Furthermore, Patent Document 2 below describes an arrangement in which, when forming two closed cross-sections by disposing a reinforcement between an outer member and an inner member, two joining flanges of the outer member and two joining flanges of the inner member are laser welded by means of two weld lines, and two joining flanges of the outer member and opposite edges of the reinforcement are laser-welded by means of two other weld lines.